1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus, and particularly to a fixing device that is compact and easy to manufacture.
2. Description of Related Art
An image forming apparatus for forming an image using an electrostatic recording system is provided with a fixing device 70 for fixing a toner image transferred onto a sheet. FIG. 8 shows a conventional fixing device 70, which is used in this type of image forming apparatus. As shown in FIG. 8, the fixing device 70 comprises a heat roller 71, a press roller 72 and a press mechanism 73 for pressing the press roller against the heat roller 71. In a fixing operation, the press roller 72 is pressed against the heat roller 71 under pressure by the pressure mechanism 73, and a sheet having a toner image thereon is passed through a gap between these rollers. The fixing device 70 is designed so that the press roller 72 is moved away from the heat roller 71 when a sheet jam occurs between the press roller 72 and the heat roller 71.
The conventional press mechanism 73 comprises an arm 74, a spring 75 and a cam 76. A rotational-shaft end portion 77 of the press roller 72 is rotatably supported by a bearing 78, which is formed at an intermediate portion of the arm 74. One end 79 of the arm 74 is rotatably mounted on a frame 80, and a spring 75 is suspended between the other end of the arm 74 and the frame 80 to press the press roller 72 against the heat roller 71 with urging the force of the spring 75. A cam 76 having a cam face, which comes into contact with the arm 74, is rotatably disposed in the vicinity of the end portion of the arm 74 at the spring side (hereinafter referred to as "spring-side end portion"). As shown in FIG. 9, when the cam 76 is rotated counterclockwise, the spring-side end portion of the arm 74 is moved against the urging force of the spring 75 in the opposite direction to the urging direction of the spring 75, that is, in a direction as indicated by an arrow of FIG. 9. With this motion, the press roller 72 is moved away from the heat roller 71.
However, since the conventional press mechanism 73 as described above is designed so that each of two arms 74 at both sides of the press roller 72 is disposed to extend to both the front and rear sides of the press roller 72, the whole length of the fixing device 70 in a sheet feeding direction is longer than the length of the press roller 71 in a radial direction, and this arrangement is an obstacle to miniaturization of the image forming apparatus. Furthermore, if a hold position of the spring 75 on the frame 80 or a securing position of the cam 76 varies, the degree of contact pressure of the press roller 72 to the heat roller 71 would also vary. Therefore, the spring 75 and the cam 76 must be secured to each other with high precision, and thus, a manufacturing process of the press mechanism is cumbersome.